Remove the database from the dsn. (#971)

See https://github.com/mongodb/mongo-php-library/issues/171

src/Jenssegers/Mongodb/Connection.php

@@ -185,7 +185,7 @@ class Connection extends \Illuminate\Database\Connection
             }
         }
 
-        return "mongodb://" . implode(',', $hosts) . "/{$database}";
+        return "mongodb://" . implode(',', $hosts) . ($database? "/{$database}" : '');
     }
 
     /**